Carbon fibrils are vermicular carbon deposits having diameters less than 500 nanometers. They exist in a variety of forms, and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces.
Tennent, U.S. Pat. No. 4,663,230, describes carbon fibrils that are free of a continuous thermal carbon overcoat and have multiple graphitic outer layers that are substantially parallel to the fibril axis. They generally have diameters no greater than 0.1 micron and length to diameter ratios of at least 5. Desirably they are substantially free of a continuous thermal carbon overcoat, i.e., pyrolytically deposited carbon resulting from thermal cracking of the gas feed used to prepare them.
Tubular fibrils having graphitic layers that are substantially parallel to the microfiber axis and diameters between 3.5 and 75 nanometers, are also described in Tenant et al., U.S. Ser. No. 871,676 filed Jun. 6, 1986 ("Novel Carbon Fibrils, Method for Producing Same and Compositions Containing Same"), Tenant et al., U.S. Ser. No. 871,675 filed Jun. 6, 1986 ("Novel Carbon Fibrils, Method for Producing Same and Encapsulated Catalyst"), Snyder et al., U.S. Ser. No. 149,573 filed Jan. 28, 1988 ("Carbon Fibrils"), Mandeville et al., U.S. Ser. No. 285,817 filed Dec. 16, 1988 ("Fibrils"), and McCarthy et al ., U.S. Ser. No. 351,967 filed May 15, 1989 ("Surface Treatment of Carbon Microfibers"), all of which are assigned to the same assignee as the present application and are hereby incorporated by reference.
Fibrils are useful in a variety of applications. For example, they can be used as reinforcements in fiber-reinforced composite structures or hybrid composite structures (i.e. composites containing reinforcements such as continuous fibers in addition to fibrils). The composites may further contain fillers such as a carbon black and silica, alone or in combination with each other. Examples of reinforceable matrix materials include inorganic and organic polymers, ceramics (e.g., lead or copper). When the matrix is an organic polymer, it may be a thermoset resin such as epoxy, bismaleimide, polyamide, or polyester resin; a thermoplastic resin; or a reaction injection molded resin. The fibrils can also be used to reinforce continuous fibers. Examples of continuous fibers that can be reinforced or included in hybrid composites are aramid, carbon, and glass fibers, alone, or in combination with each other. The continuous fibers can be woven, knit, crimped, or straight.
The composites can exist in many forms, including foams and films, and find application, e.g., as radiation absorbing materials (e.g., radar or visible radiation), adhesives, or as friction materials for clutches or brakes. Particularly preferred are fibril-reinforced composites in which the matrix is an elastomer, e.g., styrene-butadiene rubber, cis-1,4-polybutadiene, or natural rubber.
In addition to reinforcements, fibrils may be combined with a matrix to create composites having enhanced thermal, and/or electrical conductivity, and/or optical properties. They can be used to increase the surface area of a double layer capacitor plate or electrode. They can also be formed into a mat (e.g., a paper or bonded non woven fabric ) and used as a filter, insulation (e.g., for absorbing heat or sound), reinforcement, or adhered to the surface of carbon black to form "fuzzy" carbon black. Moreover, the fibrils can be used as an adsorbent, e.g., for chromatographic separations.
Fibrils are advantageously prepared by contacting a carbon-containing gas with a metal catalyst in a reactor at temperature and other conditions sufficient to produce them with the above-described morphology. Reaction temperatures are 400-850.degree. C., more preferably 600-750.degree. C. Fibrils are preferably prepared continuously by bringing the reactor to the reaction temperature, adding metal catalyst particles, and then continuously contacting the catalyst with the carbon-containing gas.
Examples of suitable feed gases include aliphatic hydrocarbons, e.g., ethylene, propylene, propane, and methane; carbon monoxide; aromatic hydrocarbons, e.g., benzene, naphthalene, and toluene; and oxygenated hydrocarbons.
Preferred catalysts contain iron and, preferably, at least one element chosen from Group V (e.g., molybdenum,tungsten, or chromium), VII (e.g., manganese), or the lanthanides (e.g., cerium). The catalyst, which is preferably in the form of metal particles, may be deposited on a support, e.g., alumina and magnesia.
The carbon fibrils have a length-to-diameter ratio of at least 5, and more preferably at least 100. Even more preferred are fibrils whose length-to-diameter ratio is at least 1000. The wall thickness of the fibrils is about 0.1 to 0.4 times the fibril external diameter.
The external diameter of the fibrils preferably is between 3.5 and 75 nanometers, i.e. determined by the particular application envisioned) have diameters within the range of 3.5-75 nanometers. Preferably a large proportion have diameters falling within this range. In applications where high strength fibrils are needed (e.g., where the fibrils are used as reinforcements), the external fibril diameter is preferably constant over its length.
Fibrils may be prepared as aggregates having various macroscopic morphologies (as determined by scanning electron microscopy) in which they are randomly entangled with each other to form entangled balls of fibrils resembling bird nest ("BN"); or as aggregates consisting of bundles of straight to slightly bent or kinked carbon fibrils having substantially the same relative orientation, and having the appearance of combed yarn ("CY") e.g., the longitudinal axis of each fibril (despite individual bends or kinks) extends in the same direction as that of the surrounding fibrils in the bundles; or, as, aggregates consisting of straight to slightly bent or kinked fibrils which are loosely entangled with each other to form an "open net" ("ON") structure. In open net structures the degree of fibril entanglement is greater than observed in the combed yarn aggregates (in which the individual fibrils have substantially the same relative orientation) but less than that of bird nest. CY and ON aggregates are more readily dispersed than BN making them useful in composite fabrication where uniform properties throughout the structure are desired. The substantial linearity of the individual fibril strands also makes the aggregates useful in EMI shielding and electrical applications.
The macroscopic morphology of the aggregate is controlled by the choice of catalyst support. Spherical supports grow fibrils in all directions leading to the formation of bird nest aggregates. Combed yarn and open nest aggregates are prepared using supports having one or more readily cleavable planar surfaces, e.g., an iron or iron-containing metal catalyst particle deposited on a support material having one or more readily cleavable surfaces and a surface area of at least 1 square meters per gram.
Preferred support materials include activated alumina or magnesia in the form of aggregates of tabular, prismatic, or platelet crystals. Such material is commercially available, e.g., from ALCOA (in the case of activated alumina) and Martin Marietta (in the case of magnesia). The activated alumina supports yield primarily combed yarn aggregates, while the magnesia supports yield primarily open net aggregates. Spherical gamma alumina particles, which yield bird nest aggregates, are available from Degussa.
It is believed that deposition of a catalyst on a support consisting of readily cleavable planar surfaces allows the fibrils to assist each other as they grow, creating a "neighbor" effect. As the catalyst particles deposited on the flat surfaces initiate fibril growth, the individual fibrils are influenced by their "neighbors". In the case of the activated alumina support, this leads to a combed yarn fibril aggregate in which the individual fibrils have the same relative orientation. The magnesia supports, although having readily cleavable planar surfaces, yield primarily lightly entangled, open net fibril aggregates because they break apart more readily into smaller particles than the activated alumina support during fibril growth, resulting in aggregates that are less ordered than the combed yarn aggregates but more ordered than the tightly entangled fibril balls. The oxide precursors used to generate the metal catalyst particles also affect the tendency of the support to break apart. The more readily the oxide and support can form a mixed oxide at the interface between them, the more likely the support is to break apart.
Further details regarding the formation of carbon fibril aggregates may be found in the disclosure of Snyder et al., abandoned U.S. patent application Ser. No. 149,573, filed Jan. 28, 1988, and PCT Application No. U.S. Ser. No. 89/00322, filed Jan. 28, 1989 ("Carbon Fibrils") WO 89/07163, and Moy et al., abandoned U.S. patent application Ser. No. 413,837 filed Sep. 28, 1989 and PCT Application No. U.S. Ser. No. 90/05498, filed Sep. 27, 1990 ("Fibril Aggregates and Method of Making Same") WO 91/05089, all of which are assigned to the same assignee as the invention here and are hereby incorporated by reference.
Fibrils are increasingly important in a variety off industrial uses and will become more so as these unique properties become better understood and exploited. While known methods of manufacture permit production of small quantities of fibrils, it is important to improve these methods, and in particular the catalysts used in those methods, to increase the yield of fibrils, to improve their quality and to lower their cost of production. It is also desirable to produce carbon fibrils of improved-purity.